"But that’s how it always is. Lorraine remembers the big words, and I remember the action. Which sort of makes sense when you stop to think that Lorraine is going to be a famous writer and I’m going to be a great actor."
Lorraine's dream is to become a writer. The dream fits her personality quite nicely. She's a subdued character. She has no desire to be the prom queen or little Miss Popular. She prefers to sit back and analyze situations and people. Being a writer would allow her to do that. She could incorporate the world that she sees and analyzes into her writing. Her writing would serve as an outlet for her sensitive and analytical nature.
John's dream is to become an actor. What John doesn't know is that he already is an actor. He's just not pulling a paycheck for it. John is a compulsive liar. He lies to everybody except Lorraine. Combined with his good looks, his ability to lie so well gives John the ability to be anybody that he needs to be in any given situation. John's dream to be actor would allow him to use the skills he already has in order to support himself financially.